The present invention relates to a line pressure control for an automatic transmission.
In automatic transmissions, a desired one of gear positions is established by delivering hydraulic fluid to selected one or ones of various friction elements (such as clutches and brakes) to hydraulically activate same, and a shift between two gear positions is effected by changing the friction element or elements to be activated. If the line pressure is excessively high, a capacity of a friction element which reveals during a shifting operation becomes excessively high, causing a great shock to occur, whereas if the line pressure is excessively low, the capacity of the friction element becomes excessively low, causing a slip to occur, thus shortening an operating life of the friction element. Thus, the line pressure has to be appropriately adjusted.
Well known is an automatic transmission which is described in the publication entitled "SERVICE MANUAL FOR AUTOMATIC TRANSMISSION OF THE RE4R01A TYPE" (A261C07) published in March 1987 by NISSAN MOTOR COMPANY, LIMITED. In this known automatic transmission, a line pressure solenoid of the duty cycle type is arranged to close or open a drain circuit of a pilot pressure to regulate a throttle pressure delivered to a pressure modifier valve where the throttle pressure is converted into a pressure modifier pressure. This pressure modifier pressure is applied to a regulator valve which generates a line pressure. The magnitude of the line pressure is controlled by varying a proportion of OFF time period to one cycle time period of a drive current which the line pressure solenoid is operated on. Since a needle valve closes the drain circuit when the line pressure solenoid is OFF while it opens the drain circuit when the line pressure solenoid is ON, the magnitude of the throttle pressure increases as the proportion of OFF time period increases. Thus, increasing the proportion of OFF time period causes the line pressure regulator to increase the magnitude of the line pressure. Various values of the proportion of OFF time period are contained in a table data in a ROM of a microcomputer of an automatic transmission control unit. Different table data are provided, one for use during a shifting operation, another for ordinary non-shifting operation, for example. These values are arranged in each table data as being retrievable by table look-up operation using a variable such as a throttle opening degree. In the present application, the term "duty" is hereinafter used to mean the proportion of OFF time period to the cycle time period and expressed in terms of percentage (%).
However, this line pressure control used in the known automatic transmission is not satisfactory in that it cannot cope with a situation where the line pressure solenoid has a manufacturing variation or the characteristic of the line pressure solenoid degrades with time or a situation where the friction element has a manufacturing variation or the frictional material of the friction element degrades with time. In the former situation, even if the line pressure solenoid is subject to the same duty, the magnitude of line pressure deviates from a target value. In the latter situation, even if the magnitude of line pressure is adjusted to the target value, the friction element does not show a desired performance characteristic. Thus, in any event, the conventional line pressure control system fails to avoid occurrence of substantial shift shock or reduction in operating life of the friction element.
In order to cope with this problem, it has been proposed to adjust the magnitude of line pressure for the next occurrence of a shifting operation as a result of learning of the present occurrence of the shifting operation as noted in U.S. patent application Ser. No. 07/289,050 or European patent application No. 88 121 587.5. According to this proposal, the shifting operation is evaluated by monitoring the inertia phase time when a gear ratio varies from one gear ratio for one gear position to another gear ratio for another gear position since the inertia phase time varies with variation in magnitude of line pressure delivered to the shifting friction element during the shifting operation. Specifically, the inertia phase time when the gear ratio, viz., a ratio of a revolution speed of a transmission input shaft (a turbine shaft) to a revolution speed of a transmission output shaft, is subject to variation is compared with a target value, and the magnitude of the line pressure for the next occurrence of the same shifting operation is varied in accordance with the result of the comparison. The target value which the inertia phase time is compared with is obtained by a table look-up operation of a data table suitable for a shifting operation conducted. This data table contains a plurality of values for shifting operations of the same kind but with different throttle opening degrees, respectively. These values in this data table are true when the automatic transmission conducts the shifting operation under predetermined circumstances. This means that they cannot be relied upon for learning when the automatic transmission conducts the shifting operation not under the predetermined conditions, causing a potential problem that the line pressure is adjusted in a wrong manner.
An object of the present invention is to improve a line pressure control of the above-mentioned type such that the potential problem mentioned above is eliminated.